The present invention relates to devices for controlling the application of power to AC electric motors; and in particular to such devices which control the starting and stopping of the motor.
A typical motor controller has thyristors which connect the motor windings to alternating current supply lines. For a three phase motor, each AC phase line usually is coupled to a separate winding within the motor by either a triac or a pair of inversely connected silicon controller rectifiers (SCR's). A circuit within the controller determines the proper time to trigger the thyristors during each half-cycle of the supply line voltage. Once a thyristor is triggered it remains in a conductive state until the alternating current flowing therethrough makes a zero crossing. By regulating the trigger time of the thyristors, the amount of time that they are conductive can be varied to control the amount of current applied to the motor and correspondingly its speed.
To start the motor, conventional motor controllers vary the thyristor trigger times to provide a gradual increase in the voltage. In doing so, the thyristors are initially triggered relatively late in each voltage half cycle so that they are conductive for only a short period. The trigger times then become progressively earlier in each half cycle to render the thyristors conductive for longer periods and apply greater amounts of voltage to the motor.
This control of electricity to the motor is graphically depicted by the waveforms in FIG. 1. The solid waveform represents the alternating voltage between a terminal for one winding of the motor and ground, while the dashed waveform represents the current through that winding. When the current reaches zero, the thyristors for that winding will automatically turn off creating a notch 8 in the voltage waveform until the thyristors are triggered again. It is noted that the voltage is not zero during this non-conductive period, due to a voltage induced across the motor winding from the back electromotive force (emf) and from the current in the other motor windings. When the motor is initially started, notch in each half cycle is relatively wide and the voltage during the notch interval has the opposite polarity to that of the supply voltage. This opposite polarity relationship exists while the motor is in a stall condition.
As the thyristors become triggered earlier in each half cycle during starting, the motor speed increases until it reaches substantially full speed, at which point the motor is no longer in a stall condition. When this occurs, the voltage across the winding during the notch has the same polarity as the supply line voltage as depicted by the dotted line 9. Eventually the thyristors are triggered immediately after the current goes to zero and are conductive during almost the entire half cycle of the voltage.
Various techniques have been devised for controlling the thyristor triggering, and thereby the application of electricity to the motor during starting. One common technique involves sensing the zero crossing of the supply line voltage and delaying an angle .alpha. between that crossing and the voltage phase angle at which the thyristor is triggered. Since the voltage waveform is periodic (see FIG. 1), the delay angle .alpha. can be expressed as an interval of time commencing at the zero crossing. By initially starting with a large delay angle .alpha. and slowing decreasing the delay, a greater amount of voltage is gradually applied to the motor until the thyristor is conductive during substantially the entire half cycle of the supply voltage. Another common technique for triggering the thyristors controls the hold-off angle .gamma. between the voltage phase angle at which the current reaches zero and the phase angle when the thyristors are triggered. As with .alpha. control, the control system expresses angle .gamma. as an interval of time from when the motor current goes to zero. Once again, by starting with a relatively large hold-off angle .gamma. and gradually decreasing it, the electricity to the motor is increased producing a commensurate increase in torque.
Both of these control techniques involve open loop systems in which a condition (zero voltage or zero current) is detected and the thyristors are triggered at an interval measured from that condition. Although the starting techniques that regulate angles .gamma. or .alpha. work well with motors driving a load that is independent of the speed, these techniques have less than optimum performance when the load torque varies as a function of speed. In motors which operate pumps for example, the load torque may increase as the square of the motor speed. In this situation, conventional "soft" motor starting techniques as described above create sudden flow surges of the pumped fluid which adversely effect the plumbing on both sides of the pump. Pipe vibration during the pump starting is a common side effect, which if left unchecked can cause ruptures over time. Therefore, it is desirable to provide a control mechanism which gradually and uniformly increases the motor torque as the motor starts preventing a sudden, sizeable speed change.
A control technique similar to .gamma. or .alpha. starting systems has been utilized to regulate the power factor of the motor under varying load conditions once the motor has reached full speed. U.S. Pat. No. 4,052,648 discloses an example of power factor regulation in which the conduction time of each thyristor switch is controlled to be inversely proportional to the phase angle difference .phi. between the zero crossing of the voltage applied to the motor and the cessation of current flow. By controlling the time at which the thyristors are triggered on, more or less current is applied to the motor which alters the phase angle difference. However, unlike .alpha. and .gamma. control, the power factor control cannot directly determine the time at which to trigger the thyristor from the sensed events, since the angle .phi. being controlled does not terminate at the triggering time. Instead, power factor control must be implemented by a closed feedback loop, wherein the error between the desired and actual values for angle .phi. is used to determine the triggering time.